1. Field of the Invention
The present invention generally relates to liquid crystal display devices and, more particularly to a liquid crystal display device in which a liquid crystal panel is illuminated from backside by a lighting device having a light source, a light-guiding member, an optical sheet, etc.
2. Description of the Related Art
A description will be given first, with reference to FIG. 1, of a conventional liquid crystal display. FIG. 1 is an exploded perspective view of a conventional liquid crystal display device.
The liquid crystal display device shown in FIG. 1 also serves as an information input unit, and comprises an information input panel 1, a bezel 2, a liquid crystal panel 3, two optical sheets 4, a light-guiding member 5, a reflective sheet 6 and a frame 7. The liquid crystal panel 3, the optical sheets 4, the light-guiding member 5 and the reflective sheet 6 are situated between the frame 7 and the bezel 2. The optical sheets 4, the light-guiding member 5 and the reflective sheet 6 together serve as a part of a back light device that illuminates the liquid crystal panel 3 from backside. The information input panel 1 is arranged on the bezel 2.
A plurality of light-emitting diodes (light source) 9 attached to a printed circuit board 8 are arranged at a side surface of the light-guiding member 5. A light emitted from the light-emitting diodes 9 is diffused by the concavo-convex pattern of the surface (reflective sheet side) of the light-guiding member 5 while transmitting inside the light-guiding member 5, and travels in a direction toward the liquid crystal panel 3. The light exiting the light-guiding member 5 on the opposite side of the liquid crystal panel 3 returns to the light-guiding member 5 by being reflected by the reflective sheet 6, and finally travels in a direction toward the liquid crystal panel 3. Thereby, the liquid crystal panel 3 is illuminated entirely from backside (back light), which brings a display on the liquid crystal panel 3 into sight.
The printed circuit board 8 having the light-emitting diodes 9 mounted thereon has through holes 8A on opposite ends thereof so that the printed circuit board 8 is positioned with respect to the frame 7 by fitting the through holes on projections 7A of the frame 7. Additionally, the light-guiding member 5 is positioned with respect to the frame 7 by the outer periphery thereof fitting into a side surface or a recess of the frame 7. Thereby, the light-emitting diodes 9 attached to the printed circuit board 8 are positioned in a state where the light-emitting diodes 9 align along the side surface of the light-guiding member 5. The light of the light-emitting diodes 9 can be guided to the light-guiding member 5 by the light-emitting surfaces of the light-emitting diodes 9 facing the side surface of the light-guiding member 5. An input part 8B extends from the printed circuit board 8 so that an electric current for illumination is supplied to the light-emitting diodes 9 through the input part 8B.
It should be noted that a substrate 3A extends from one side of the liquid crystal panel so as to input a drive signal to the liquid crystal panel 3. Additionally, a board 1A extends from the information input panel 1 so as to output a signal. Moreover, the information input panel 1 is not indispensable for the liquid crystal display device, and there is no need to provide the information input panel 1 if only a display function is needed and the information input function is not needed.
With the above-mentioned structure, the printed circuit board 8 to which the light-emitting diodes 9 are fixed is positioned with respect to the frame 7, and the light-guiding member 5 is also positioned with respect to the frame 7. According to such a positioning structure, a positioning error of the printed circuit board 8 with respect to the frame 7 and a positioning error of the light-guiding member 5 with respect to the frame 7 are added, which may form a large air gap between the light-emitting diodes 9 and the light-guiding member 5. If a large air gap is formed between the light-emitting diodes 9 and the light-guiding member 5, the light from the light emitting diodes 9 scatters outside through the air gap, which raises a problem in that the light from the light-emitting diodes 9 cannot be efficiently introduced into the light-guiding member 5.
Moreover, if the number of light-emitting diodes 9 is not sufficient as shown in FIG. 3, there may be a problem in that an uneven illumination occurs in the light emitted from the entire light-guiding member 5 when the light of the light-emitting diodes 9 is input to the light-guiding member 5 since an interval between the light-emitting diodes 9 is too large.